1. Field of the Invention
The present invention relates to radiation-curable coating compositions having improved properties, such as adhesion to the substrate and release properties which are stable over time.
2. Background of the Invention
Polysiloxanes containing acrylate acid ester groups (acrylate groups) have become established as binders which can be cured under high-energy radiation for applications such as printing inks and for producing film-forming binders or for coating materials for surfaces of plastic, paper, wood and metal. Curing takes place in particular by UV radiation (following the addition of known photoinitiators, such as benzophenone and its derivatives, for example) or by means of electron beams.
3. Description of the Related Art
Acrylate-modified organosiloxanes are described in numerous patent documents. The following patents and applications are cited as representatives of the prior art.
Organopolysiloxanes in which the acrylate-containing organic groups are connected to the polysiloxane framework by way of an Si--O--C bond can be prepared in accordance with a process of DE-C-27 47 233 by reacting --COH-containing (meth)acrylic esters with organopolysiloxanes which have SiX groups (X=alkoxy, hydroxyl or chlorine) wherein the organopolysiloxanes used are of the formula ##STR1##
and the siloxane molecule has 3 to 100 silicon atoms and, as the (meth)acrylic ester, using pentaerythritol tri(meth)acrylate, from 0.05 mol to equimolar amounts of the pentaerythritol ester being employed relative to COH and SiX groups.
In a modification of this process the procedure in accordance with DE-C-29 48 708 is to react organopolysiloxanes of the formula ##STR2##
(R.sup.1 =alkyl of 1 to 4 carbon atoms, vinyl and/or phenyl with the proviso that at least 90 mol % of the groups R.sup.1 are methyl; a=from 1.8 to 2.2; b=from 0.004 to 0.5) first of all with at least twice the molar amount, based on SiCl groups, of a dialkylamine whose alkyl groups each have 3 to 5 carbon atoms and in which the carbon atoms adjacent to the nitrogen each carry not more than one hydrogen atom and then reacting the product of this first reaction with at least equimolar amounts of pentaerythritol triacrylate or pentaerythritol trimethacrylate and, subsequently, in a manner known per se, separating off the product of the process from solid constituents suspended in it.
Organopolysiloxanes, in which the acrylic ester-containing organic groups are connected to the polysiloxane framework by way of Si--C bonds can be prepared, for example, by subjecting allyl glycidyl ether or another suitable epoxide having an olefinic double bond to an addition reaction with a hydrosiloxane and, following the addition reaction, esterifying the epoxide with acrylic acid to open the epoxide ring. This procedure is described in DE-C-38 20 294.
A further possibility for the preparation of acrylate-modified polysiloxanes with Si--C linkage of the modifying group(s) is to subject an alcohol having an olefinic double bond, such as allyl alcohol, to an addition reaction with a hydrosiloxane in the presence of a platinum catalyst and then to react the OH group of this alcohol with acrylic acid or with a mixture of acrylic acid and other, saturated or unsaturated acids. This procedure is described, for example, in DE-C-38 10 140.
When considering unbranched organopolysiloxanes, modification can take place at the two terminal siloxydimethyl units or at one or more siloxymethyl units within the siloxane chain. Accordingly, one talks of terminally or laterally modified polysiloxanes. The modification can also be both terminal and lateral; in the text below, such modifications are included among the laterally modified products.
It has been found in practice that abhesive coatings produced with terminally (meth)acrylate-modified organopolysiloxanes on sheetlike supports, for use in adhesive tapes or label laminates, for example, feature low release values, which are stable over time, since such polysiloxanes have a high silicone character which is not interrupted by organic modifications within the siloxane chain. To achieve good adhesion to the substrate, however, a relatively high modification density with reactive groups is necessary. This is difficult to achieve with terminally modified siloxanes, since the siloxane chain would have to be shortened correspondingly. Such short-chain terminally modified polysiloxanes are comparatively expensive, so their use would be uneconomic.
Laterally modified silicone (meth)acrylates, on the other hand, can be varied over wide ranges in their modification density independently of the molecular weight. Since lateral (meth)acrylate groups are known to be less reactive owing to their steric hindrance, there is a risk that not all groups will react completely during the polymerization. Such residual, uncrosslinked acrylate groups can then enter into follow-on reactions--for example, with the pressure-sensitive adhesive of a label laminate--which would, over time, lead to an increase in the release values.